1. Field of the Invention
The present invention relates to a color thermal printer for recording a full-color image in a frame sequential fashion. More particularly, the present invention relates to a paper transport device for a color thermal printer, wherein recording paper is transported alternately in a recording direction and in the opposite direction through a single thermal head, and one color frame is recorded during one transport of the recording paper in the recording direction.
2. Background Arts
In a color thermal printer, at least three color frames are sequentially recorded in a recording area on the recording paper. To prevent color failure, accurate registration between the color frames is necessary, so the positioning of the recording area relative to a heating element array of the thermal head should also be accurate. Especially in a color thermal printer where the recording paper is transported repeatedly through a single thermal head, it is necessary to control the paper transport amount relative to the thermal head with high accuracy. For this reason, a transport roller device consisting of a capstan roller and a press roller is disposed in proximity to the thermal head in this type of color thermal printer. For more accurate and stable paper transport, it has been suggested to turn the recording paper around the capstan roller.
As an array of heating elements of the thermal head is directly or indirectly pressed onto the recording paper while heating the recording paper, the recording paper is heat-deformed or curled to some extent. Particularly where the recording paper turns around the capstan roller, the curl becomes so large that it can cause variation in transport amount of the recording paper per a rotational angle of the capstan roller. Especially in a color thermal printer having a single thermal head, as the color recording paper is heated at least three times for recording three color frames, the degree of curling increases gradually.
Because thermosensitive color recording paper has coloring layers sequentially overlaid on a support material, and the bottom coloring layer nearest to the support material has the lowest heat sensitivity, the bottom coloring layer is recorded at last with the largest heat energy among these coloring layers. Consequently, the degree of curling is getting larger as the recording proceeds. The more the recording paper curls, the more the transporting amount per a rotational angle of the capstan roller varies. In addition, the support material of the thermosensitive color recording paper is so stiff that it amplifies the influence of curling on the transporting amount by the capstan roller.
Moreover, it is necessary for the thermosensitive color recording paper to optically fix or deactivate the top and second from the top coloring layers after each recording of corresponding color frames, so as not to develop color any more during the next color recording. In a thermosensitive-type printer where optical fixing lamps for the two colors are provided side by side along a paper transport path, the thermosensitive recording paper is transported by a different amount for the optical fixing of one color from the other color, in order to exposure the same recording area to the light of each fixing lamp. The difference in the transport amount necessary for the optical fixing of one color from the other makes it difficult to measure the variation in the transporting amount of the curled recording paper by the capstan roller.